1. Field of the Invention
The present invention relates to a crystal oscillator circuit using a quartz vibrator and an inverter.
2. Description of the Related Art
FIG. 1 is a view showing the basic configuration of a crystal oscillator circuit using a quartz vibrator and a CMOS inverter.
This circuit is provided at the outside of the chip with a quartz vibrator X1, two capacitors C1 and C2 connected between its two ends and the ground, and a stabilizing resistor element R2 for stabilizing the oscillation operation, one end of which is connected to one end of the quartz vibrator X1 and one electrode of the capacitor C2.
In the chip, the circuit is provided with a CMOS inverter 1 for voltage amplification parallel to the quartz vibrator X1. The input side node n1 of the CMOS inverter 1 is connected through an input-output terminal T1 to the other end of the quartz vibrator X1 and one electrode of the capacitor C1 outside the chip, while the output side node n2 of the CMOS inverter 1 is connected through the input-output terminal T2 to the other end of the stabilizing resistor element R2 outside the chip. Further, the output side node n2 and input side node n1 of the voltage amplification CMOS inverter 1 have connected between them a feedback resistor element R1 parallel to the CMOS inverter 1. The input of the output inverter 2 is connected to the output side node n2.
The resistance of the feedback resistor element R1 is selected to be a value greater than the resistance of the stabilizing resistor element R2.
In this crystal oscillator circuit, a sine wave signal is generated by the resonance of the quartz vibrator X1 and the two capacitors C1 and C2. This sine wave signal is amplified by the CMOS inverter 1 and then shaped in its waveform by the inverter 2 to give the output OUT.
Crystal oscillator circuits using quartz vibrators and CMOS inverters in this way are widely used as oscillators with a high degree of stability of frequency with respect to changes in the ambient temperature and power-supply voltage.
In the above-mentioned oscillator circuit, however, since the output side node n2 of the CMOS inverter 1 is connected to the outside input-output terminal T2, noise enters from the outside and therefore it suffers from the disadvantage of generation of unnecessary pulses at the output node.
Further, since the oscillation waveform of the oscillator circuit, that is, the waveform at the nodes n1 and n2, became a sine wave, a large short current flowed near the logical threshold value in the voltage amplification CMOS inverter 1 to which the sine wave was input, which became a major disadvantage when operating at a low power consumption.
In the same way, since the amplitude of the sine wave of the node n1 does not swing fully between the power-supply voltage and the ground for example due to the loss resistance of the stabilizing resistor element R2 and the quartz vibrator X1, the short current continues to flow at all times during operation to the voltage amplification CMOS inverter 1, which became a major disadvantage when operating at a low power consumption.